This invention relates generally to a low drop out (LDO) voltage regulator that enables reliable long-term operation over a wide range of input voltages. More particularly, the invention relates to a circuit architecture that enables reliable, long-term, voltage regulation over a wider range of input voltages.
A low dropout or LDO regulator is a voltage regulator which has a very small input-output differential voltage. The main components of a LDO voltage regulator are a power transistor and a differential amplifier (sometimes known as an error amplifier). One input of the differential amplifier monitors a portion of the output, as determined by a ratio of two resistances. A second input to the differential amplifier is from a stable voltage reference (commonly referred to as the bandgap reference). When the output voltage rises too high relative to the reference voltage, the drive to the power transistor changes so as to maintain a constant output voltage.
A voltage regulator's dropout voltage determines the lowest usable supply voltage. If, for example, the LDO voltage regulator has a dropout voltage around 700 mV (0.7V), a 3.3V output would require the input to be at least 4.0V. Such a LDO voltage regulator may be specified to provide a fixed 3.3V output with a 4.0V to 5.5V input voltage range.
A LDO voltage regulator's output voltage varies in accordance with several factors. For example, output voltage of an LDO voltage regulator can be affected by variation in the temperature of the constant voltage reference source and the differential amplifier characteristics, as well as variation in the tolerances of individual sampling resistors.
The use of small geometry and low-voltage devices (i.e., devices that reliably operate when the voltage across any two transistor terminals is less than a relatively low maximum voltage) is the trend in advanced integrated circuits (ICs). These low-voltage digital-logic devices consume less power and can be reliably operated at higher clock rates. Accordingly, low-voltage devices are used in a number of battery-operated portable electronic systems. Intermediate voltage-level devices (i.e., devices that reliably operate when the voltage across any two transistor terminals is less than approximately 3V) are generally used in ICs that require analog functions. Even higher voltage levels are required by some circuits used in both analog and digital functional blocks related to system interfaces and other functions, such as those required by wireless communication devices. One way to accommodate these higher voltages is to use transistors designed to operate reliably at corresponding higher voltage levels. For example, transistors where the voltage across any two transistor device terminals can be 5V without reliability issues (i.e., 5V transistors) can be used to implement functions over a range of voltages from 0V to about 5V. This solution requires a second IC or the addition of devices designed to manage these higher voltages when the bulk of IC functionality is provided via a first IC that uses lower-voltage devices. Accordingly, ICs using higher-voltage transistors in addition to low-voltage devices result in increased cost and complexity for the final product.
Typically, IC manufacturers do not provide a product that combines low-voltage digital transistors, 3V analog input/output transistors and 5V or higher analog/power transistors using a single manufacturing process. Accordingly, there would be a significant cost associated with using and developing a semiconductor wafer manufacturing process that could provide the desired combination of transistors on a single IC.
Therefore, it would be desirable to provide a low cost, reliable and integrated LDO voltage regulation solution that can be implemented using existing semiconductor manufacturing process technologies.